Multichip modules that incorporate flip chip design implementations are increasingly being used for a large variety of applications. In many cases, it is advantageous to utilize established integrated circuit designs for a variety of operations and/or functions. Accordingly, the established designs found in manufactured die are therefore combined on a single board or multichip module. For a variety of manufacturing considerations, however, the various integrated circuit devices, and more particularly flip-chip devices, are not always installed into the multichip modules at the same time. For example, different flip chip devices may be manufactured by different fabrication facilities and, accordingly, are added to the multichip modules at different times. There are multiple techniques that can be used to attach a die to a board or module including wire bonding, die attach film and soldering at the different times. Such applications are often used though not always appropriate for all applications. For example, a wire bond solution may not be appropriate in situations where footprint requirements necessitate efficient integrated circuit (IC) real estate usage.
Solder alloys and pastes are often used in conjunction with a reflow oven or furnace to bond a die to a board. Using solder in either form is beneficial for bonding one die to a board, such as a ceramic or substrate board or metal lead frame, in certain circumstances because a reliable bond is formed once the solder cools after being removed from the reflow oven or furnace. Typically, the solder alloy or paste will flow when the device and board are placed in the reflow oven and will then cool to bond the die to the board and, if desired, to form an electrical connection between the device and board. For the case of a single die, this approach is also beneficial in that the die may be removed if faulty by subsequently reflowing the die and board in the reflow oven or furnace. This process typically requires, however, for sufficient spacing between die and between bond pads of the die so that the solder, when melted during reflow, does not spread to contact other bond pads of a current die being reflowed or of another die.
Another issue with using solder alloys or paste, however, is that the solder will melt and reflow when the board and another die are placed in the reflow oven to bond the new die to the board. Accordingly, the bond that adheres the originally bonded die may fail while the solder is being melted in the reflow oven or furnace to bond the subsequent die. One approach to solve this problem has been to use different metal alloys for the first die so that the metal alloys bonding the die have a higher reflow temperature than what is needed for re-flowing the solder alloys. Two metals that have been used to this end include lead and gold because they have higher melting temperatures. Lead, however, is known to cause health issues and is not always desirable. Gold, on the other hand, is expensive and drives up product cost. To form a gold layer on a die or board that is to be subsequently etched away and then melted during a reflow is wasteful of a precious metal unless such an approach is necessary. Accordingly, solder is often used despite its limitations.
The use of the same reference symbols in different drawings indicates similar or identical items. Unless otherwise noted, the word “coupled” and its associated verb forms include both direct connection and indirect electrical connection by means known in the art, and unless otherwise noted any description of direct connection implies alternate embodiments using suitable forms of indirect electrical connection as well.